I think it is a good idea that you are aiming for standard awards to Africa. However, just keep in mind there are many times (a week to a few day lead) when the saver awards pop up.
With that said, I am sure everyone here will mention credit card spending as a way to gain mileage. However, it would be best to get an idea of how much money you could spend on CCs in a year. If you need help meeting these spends, check the thread labeled "getting spend up without the mint" alternatively PM me.
Anyway for United points, I suggest the following methods for both you and your wife:
Chase INK Bold & Chase INK Plus each card with 5k spend nets you 50k points, so you and your wife with 20k spending could net 220k (200k signup bonus + 20k regular points) Ultimate reward points, which are directly transferable to United.
The other card you would be interested in is the Chase Sapphire Preferred card (which I signed up and was approved for today). This card will net you each 43k (40k bonus + 3k regular) ultimate reward points after 3k in spending.
With those methods you will net 306k united points.